In 1959, researchers Peter Karlson and Martin Luscher coined the term “pheromones” to designate substances secreted by an individual that trigger a stereotypical (unlearned) behavioral or endocrine reaction in their conspecifics. Among them, the most popular were (and are) the so-called sexual pheromones, described for the first time in female silk moths (Bombix mori). Without the need for prior experience, minute amounts of this female pheromone immediately attract males of their species, thus facilitating reproduction.
The existence of human pheromones was immediately raised, which would inevitably attract individuals of the other sex. Colognes, perfumes and deodorants with these supposed substances can be found on the internet… to feed the lubricious dreams of the ignorant.
The fact is that until the beginning of the 21st century, sexual pheromones were not identified in any species of mammals. It was difficult because, to prove that a substance acts as such, it must be proven that it is attractive to individuals of the opposite sex without previous experience with them or their smells. That is, in individuals deprived of social contact until the moment of the experiment.
In the early 2000s, in our Comparative Functional Neuroanatomy laboratory at the University of Valencia we raised female mice without males or their scents (females alone, with their mother and sisters) until adulthood. We subjected these “chemically virgin” females to a simple choice test between sawdust from boxes of adult males – with an intense characteristic odor – and other types of bedding: clean sawdust and bedding obtained from boxes of females or from castrated males.
Well, females explored male sawdust more when we allowed direct contact of their snout with it. On the contrary, if a perforated platform allowed the smell to pass through but prevented contact with the sawdust, there was no preference.
The reason for these results is that the mouse’s male-attractive pheromone is a non-volatile urinary protein, of a type known generically as lipocalins. Lipocalins have “hydrophobic pockets” in which they retain – and protect from degradation – volatile lipid odorous compounds.
Years later, Jane Hurst and Rob Beynon, from the University of Liverpool, identified lipocalin in males that triggers sexual attraction. They named it darcina, in honor of Mr. Darcy, the handsome male protagonist of Jane Austen’s novel Pride and Prejudice.
It has been proven that darcin is not detected by the olfactory epithelium of the mouse, but by its vomeronasal organ (VNO) or Jacobson’s organ, whose neurons express specific membrane receptors for different pheromones. It is therefore accepted that the VNO is the organ responsible for detecting pheromones.
Thanks to Charles Wysocki and George Preti asking themselves this same question, we know that we have axillary apocrine sweat glands that produce odorous fatty acids and androgen derivatives (androstenone, androstenol, and androstadienone). These are good candidates for male sex pheromones, since they are more abundant in men than in women, and they appear in urine and armpit sweat only after puberty. Furthermore, these compounds are associated with lipocalins similar to darcin.
However, various reasons lead us to think that these substances do not act as pheromones:
On the other hand, there are numerous studies on the recognition of the sex of T-shirt wearers by the smell of sweat that they give off. There is also evidence that six-day-old babies, not earlier, recognize the smell of their mother, and vice versa (there is very little information about fathers).
However, these data only demonstrate learning (odor memory) dependent on social experience. Nothing to do with the stereotyped, unlearned response to pheromones.
Other studies suggest that lactation could indeed be mediated by pheromones. After birth, the baby shows the so-called sucking reflex, on which its survival depends: it takes it to its mouth and sucks the nipple of the mother’s breast with relish. Researcher Robyn Hudson studied this reflex in rabbits, whose offspring struggle with their numerous siblings to find their mother’s nipples during their mother’s short visits (five minutes) to the burrow, and proposed the existence of mammary pheromones that would mediate this behavior.
Years later, Benoist Schaal detected a substance in the rabbit’s milk, 2-methylbut-2-enal, that is attractive to neonatal pigs. It immediately induces in them a “suction reflex” directed at the pipette that contains it. Could there be a human mammary pheromone, similar to that of the rabbit? It seems so, but it would not be secreted by the mammary glands, but by glands of the mammary areola. These do not produce milk, but a transparent liquid.
Interestingly, the number of areolar glands correlates with the ease with which the baby latches onto the breast, and primiparous women without these glands have great difficulty initiating breastfeeding.
On the other hand, in the first attempt at breastfeeding, newborn babies show an increase in respiratory rate and a reflex approach towards the secretion of the areolar glands. If we identified the pheromone of the areolar glands, perhaps we could use it to more easily achieve the miracle of breastfeeding. An interesting challenge for future science.
This article was originally published on The Conversation.
Ferran Martínez-García is a Professor of Cellular Biology at the Predepartmental Unit of Medicine; Faculty of Health Sciences, Universitat Jaume I
